1. Field of the Invention
The present invention relates to an apparatus and method for use with a corrosion monitoring and/or mitigation system. More specifically, the invention relates to an apparatus and method for monitoring cathodic protection while supplying cathodic protection power to an object being protected. Yet more specifically, the invention relates to a system for determining electrolyte corrosivity and optimum site specific cathodic protection operating levels.
2. Description of the Related Art
Cathodic protection systems mitigate corrosion of metallic objects that are partially or wholly submerged in mediums (such as soil or water) where they are exposed to corrosive electrolytes. For example, points or sections on pipelines immersed subsea or buried under the earth's surface can experience an electrical potential difference from other portions of the pipeline because of characteristics in the medium, or differing characteristics in the pipeline itself. Corrosion results when the potential difference causes electron flow between the pipeline sections of different potential. Cathodic protection involves placing an anodic material in the common electrolyte with a corroding metallic surface and providing an electrical connection between the anodic material and the corroding metal. The anodic material can be galvanically anodic, or forced to be anodic with the use of an external dc power supply. The surface that is more anodic experiences corrosion, and the surface that is less anodic (or more cathodic) does not corrode. The pipeline surface then becomes more negatively polarized than it previously was. The steel will not break down into Fe+ ions and electrons when there is already an excess of electrons on the steel surface. The steel surface in this condition is cathodic relative to the anode material. If correctly applied, all corrosion occurs on the anode material.
Current is sometimes impressed onto the protected metallic object, provided via electrical power, to make the protected metallic object more cathodic (electrically negative) than the anode. Monitoring the effectiveness of the cathodic protection is generally performed by measuring the electrical potential of the protected metallic object relative to a reference electrode that is set in the water or soil. The electrical potential can be measured when the current is being impressed onto the protected metallic object (referred to as on potential) or not being impressed (referred to as off potential). The resistance of the soil or water and protected metallic object introduce a measurement error (IR error) due to a corresponding voltage drop from the resistance. Interrupting the current supply to the protected metallic object and instantaneously measuring the potential between the protected metallic object and reference electrode (referred to as instant off potential) yields a value for potential void of IR error.
Cathodic protection for well casings and long pipelines is typically applied in an impressed current configuration, e.g., from an external DC power supply. Impressed current cathodic protection involves the introduction of a conductive material (typically cast iron rods) buried in the ground and electrically connected to the positive (anode) terminal of an external DC power supply. The negative (cathode) terminal of the power supply is connected to the structure to be cathodically protected.